The present invention relates to wing trailing edge flaps for aircraft and, more particularly to a structure which supports the flap by two elements, a linkage system and a track connected to the wing, moving the trailing edge flap system between the extended and retracted positions.
The aerodynamic design of modern aircraft wings is a compromise between many conflicting requirements, thus limiting near optimum aerodynamic performance to a small portion of their flight envelope. Obviously, great emphasis must be placed on cruise configuration, as this is the regime most frequently experienced. However, modern wings must be configured to include high lift devices, such as flaps, which are, in affect, extensions of the wing. Flaps are used to enhance lift during takeoff and landing and are retracted when the aircraft is in a cruise configuration. Drag, weight, stress levels, system flexure, and system complexity are all important factors in flap drive systems.
While the flap drive system is, to various degrees, dependent upon and determined by the lofted shape of the flap, the location of the spoiler, the location of the wing fixed trailing edge, the relationship of the flap to the trailing edge gaps and overhangs (gap and overhang may be defined respectively as the aerodynamically described distance between the wing fixed trailing edge and the lofted surface of the flap, and the distance between the trailing edge of one airfoil and the leading edge of another airfoil, e.g., a trailing edge of the wing and the leading edge of the flap), and the aerodynamic requirements dictating flap extension and flap angle at takeoff, landing and cruise are the most critical requirements. Many different approaches to the drive mechanism may be used and have been used in the past.
The simplist system is a simple hinge contained within the wing structure as shown in U.S. Pat. No. 3,594,851, issued to Swatton. A simple pivot flap may be enhanced by a slaved slot door which acts as a vane to direct the boundary layer air as shown in U.S. Pat. No. 2,920,844, to Marshall, et al. In most modern aircraft, simple systems are not satisfactory because the lift enhancement required dictates either a pivot point which is far below the wing surface or requires translation of the flap as well as rotation. In order to translate as well as rotate, a track may be used to support the flap; a linkage driving a flap on a track was taught in U.S. Pat. No. 2,271,763 to Fowler. In this reference, the flap is totally supported by the track. In some cases, the tracks have been attached to the wing with the carriage attached to the flap which rolls on a fixed track as taught in U.S. Pat. No. 1,670,852 to Fowler. Alternately, the track may be attached to the flap as taught in U.S. Pat. No. 3,438,599 to Welzen. In this rather interesting reference, the fixed carriage is attached to the fuselage at the inboard end of the flap and the outboard of the flap is supported by a very long simple hinge.
A two-track arrangement with a screw jack actuator, where the programming track is attached to the flap rather than the wing, is taught in U.S. Pat. No. 2,836,380, issued to Pearson. Two converging tracks are taught in U.S. Pat. No. 2,426,785 to Nauman, while no drive is shown. Another two-track arrangement with a curved track to program the flap is taught in U.S. Pat. No. 2,677,512, issued to Kirkbride, et al. U.S. Pat. No. 2,620,147 to Butler, et al teaches a two-track support with a ball screw drive where one track articulates. The screw translates in the nut. U.S. Pat. No. 2,609,166 to Bellam is another variation of two tracks and a screw jack drive where the two tracks converge and the nut translates on the screw jack.
U.S. Pat. No. 3,853,289 to Nevermann, et al is representative of the class of mechanisms that translates and rotates the flap as in the track systems; however, the support and motion is totally provided by a linkage system and a rotary drive. This linkage system is a textbook six-bar linkage system of the Stephenson I type.
Two-track systems are generally complex and difficult to install and maintain tolerances. Additionally, all of these use a linear-type actuator like a ball screw drive. The six-bar linkage system of U.S. Pat. No. 3,853,289 provides a compact linkage mechanism; however, because of the number of pins in the linkages system, the system flexure is very high. It is difficult to maintain the required rigidity.
In summary, initial flap systems employed simple hinge systems. Aerodynamic requirements then became more complex and dictated flap extension as well as rotation, and the art, generally, went to the two-track system. The nature of these systems is such that the support points, which is a set of wheels on each track, are very close together imposing a large moment load which results in high wheel loads because of the close proximity of the two support points. An all-linkage system separates the support points but has an inherent flexture problem because of the number of links and, particularly, pivot points are subject to wear. It is important to note that many different drive mechanisms can drive the same flap system. The reason for that is, as noted above, it is axiomatic that you cannot meet all of the aerodynamic requirements. The flap cruise position, which is essentially the stowed position, along with the flap position at takeoff and landing, as well as gaps and overhangs in these two latter positions determine the locus of points through which the flap must travel. The points are then picked to support the flap and the locus of these support points is then established by moving the flap along its previously established course. The locus of points and the two support points, then, of course, establish what the mechanism needs to do. The task is to invent a mechanism which best moves the flap through its previously established course at minimum cost and weight.
It is an object of this invention to produce the most efficient kinematic chain with the optimum number of degrees of freedom to best drive the flap through the aerodynamically determined optimum locus of points for cruise, landing, and takeoff positions as well as producing optimum gaps and overhangs at the takeoff and landing positions. Its emphasis is on simplicity and light weight, avoiding the complexities of the two-track system, and the lack of rigidity of the trackless six-bar linkage system.